Many automotive vehicles have rear wheel drive. Additionally many automotive vehicles with rear wheel drive have selective four wheel drive capabilities which are actuated upon operator command or automatically actuated in response to a slip condition of the powered rear wheels of the vehicle. To provide four wheel drive capabilities in the vehicle, there is provided a transfer case. The transfer case is typically provided at an output of the vehicle's transmission. The transfer case often has an outer housing provided by front and rear shells. A front axle output shaft is bearingly supported in the front and rear shells and rotates about a secondary axis. A front axle output sprocket is torsionally connected on the front axle output shaft. A main input shaft rotates about a coaxially aligned primary axis generally parallel to the secondary axis. The main input shaft is bearingly supported directly or indirectly by the front and rear shells. The main input shaft is torsionally connected with an output shaft of the transmission of the vehicle and with a rear axle (via a prop shaft and differential) of the vehicle. A main sprocket is provided that is rotatably mounted on the main input shaft by a bushing. A belt, chain or gear provides a power transmission device located between the main input shaft and front axle output shaft. The power transmission device torsionally connects the main sprocket with the front axle output sprocket.
A clutch assembly includes a clutch housing coaxial with the primary axis and that is torsionally connected to the main sprocket. A hub is provided spline connected to main input shaft. The hub has a splined outer diameter surface. A friction pack is provided having a first plurality of friction members torsionally connected to the clutch housing. The first friction members are axially intertwined with a second plurality of friction members spline connected with the hub.
A pressure plate is provided that is spline connected to the hub. The pressure plate is used to selectively engage and disengage the friction pack. A first ball ramp is positioned surrounding the main input shaft axially spring biased away from the pressure plate. A second ball ramp is provided for axial displacement relative to the first ball ramp. Relative displacement of the ball ramps causes the pressure plate to compressively activate the friction pack to torsionally connect the main input shaft and the hub with the clutch housing and main sprocket. The above noted torsional connection creates driving continuity transferring torque to front axle output shaft from the main input shaft.
Many transfer cases have high and low gear modes (or ranges) of operation. High mode is for normal operation, low gear is for instances the vehicle is in very difficult road conditions wherein maximum torque is required to move the vehicle. To allow towing of the vehicle without churning lubricant fluid in the transmission, the transfer case is typically also provided with a neutral mode of operation.
To reduce the spatial profile of transfer cases and to lower the weight of the transfer case a new design has been brought forth by Pritchard U.S. Pat. No. 9,447,873 (the disclosure of which is incorporated by reference herein). Pritchard has an electrical clutch actuating device or system and a range shifting assembly system that uses a common actuating motor. The Pritchard clutch actuating system and the range shifting assembly are coaxial with one another. Upon an inadvertent shift into the neutral mode due to a mechanical failure, the transfer case can possibly cause the transmission to be isolated from the front and rear axles and essentially place both axles in neutral, allowing the vehicle to move unless the vehicle brakes are applied. It is desirable to provide a Pritchard type transfer case wherein if there is a failure of the range shifting assembly, the transfer case will stay in either the high gear mode or low gear mode of operation.